A Type II civilization—also called a stellar civilization—can harness the total energy of its planet's parent star (the most popular hypothetical concept being the Dyson sphere—a device which would encompass the entire star and transfer its energy to the planet(s)).

A Type III civilization—also called a galactic civilization—can control energy on the scale of its entire host galaxy.[2]

The scale is hypothetical, and regards energy consumption on a cosmic scale. Various extensions of the scale have since been proposed, including a wider range of power levels (types 0, IV and V) and the use of metrics other than pure power.

In 1964, Kardashev defined three levels of civilizations, based on the order of magnitude of power available to them:

Type I

Technological level of a civilization that can harness all the energy that falls on a planet from its parent star (for Earth–Sun system, this value is close to 7x1017 watts), which is more than five orders of magnitude higher than the amount presently attained on earth, with energy consumption at ≈4×1019erg/sec (4 × 1012 watts).[1] The astronomer Guillermo A. Lemarchand stated this as a level near contemporary terrestrial civilization with an energy capability equivalent to the solar insolation on Earth, between 1016 and 1017 watts.[3]

Type II

A civilization capable of harnessing the energy radiated by its own star—for example, the stage of successful construction of a Dyson sphere—with energy consumption at ≈4×1033 erg/sec.[1] Lemarchand stated this as a civilization capable of utilizing and channeling the entire radiation output of its star. The energy utilization would then be comparable to the luminosity of our Sun, about 4×1033 erg/sec (4×1026watts).[3]

Type III

A civilization in possession of energy on the scale of its own galaxy, with energy consumption at ≈4×1044 erg/sec.[1] Lemarchand stated this as a civilization with access to the power comparable to the luminosity of the entire Milky Way galaxy, about 4×1044 erg/sec (4×1037 watts).[3]

Energy consumption estimated in three types of civilizations defined by Kardashev scale

At the current time, Humans have not yet reached Type 1 civilization.
Physicist and futurist Michio Kaku suggested that humans may attain Type I status in 100–200 years, Type II status in a few thousand years, and Type III status in 100,000 to a million years.[4]

Carl Sagan suggested defining intermediate values (not considered in Kardashev's original scale) by interpolating and extrapolating the values given above for types I (1016 W), II (1026 W) and III (1036 W), which would produce the formula

K=log10⁡P−610{\displaystyle K={\frac {\log _{10}P-6}{10}}},

where value K is a civilization's Kardashev rating and P is the power it uses, in watts. Using this extrapolation, a "Type 0" civilization, not defined by Kardashev, would control about 1 MW of power, and humanity's civilization type as of 1973 was about 0.7 (apparently using 10 terawatt (TW) as the value for 1970s humanity).[5]

In 2012, total world energy consumption was 553 exajoules (7020553000000000000♠553×1018 J=153,611 TWh),[6] equivalent to an average power consumption of 17.54 TW (or 0.7244 on Sagan's Kardashev scale).

In 2015, a study of galactic mid-infrared emissions came to the conclusion that "Kardashev Type-III civilizations are either very rare or do not exist in the local Universe".[7]

On October 14, 2015, the detection of an unusual light curve for star KIC 8462852 raised speculation that a Dyson Sphere (Type II civilization) may have been discovered.[8][9][10][11][12] The SETI Institute's initial radio reconnaissance of KIC 8462852, however, found no evidence of technology-related radio signals from the star.[13][14][15]

In 2016, Paul Glister, author of the Centauri Dreams website, described a signal apparently from the star HD 164595 as requiring the power of a Type I or Type II civilization, if produced by extraterrestrial lifeform.[16] However, in August 2016 it was discovered that the signal's origin was most likely a military satellite orbiting the Earth.[17]

Large-scale application of fusion power. According to mass–energy equivalence, Type I implies the conversion of about 2 kg of matter to energy per second. An equivalent energy release could theoretically be achieved by fusing approximately 280 kg of hydrogen into helium per second,[18] a rate roughly equivalent to 8.9×109 kg/year. A cubic km of water contains about 1011 kg of hydrogen, and the Earth's oceans contain about 1.3×109 cubic km of water, meaning that humans on Earth could sustain this rate of consumption over geological time-scales, in terms of available hydrogen.

Antimatter in large quantities would have a mechanism to produce power on a scale several magnitudes above our current level of technology.[citation needed] In antimatter-matter collisions, the entire rest mass of the particles is converted to radiant energy. Their energy density (energy released per mass) is about four orders of magnitude greater than that from using nuclear fission, and about two orders of magnitude greater than the best possible yield from fusion.[19] The reaction of 1 kg of anti-matter with 1 kg of matter would produce 1.8×1017J (180 petajoules) of energy.[20] Although antimatter is sometimes proposed as a source of energy, this does not appear feasible. Artificially producing antimatter—according to current understanding of the laws of physics—involves first converting energy into mass, which yields no net energy. Artificially created antimatter is only usable as a medium of energy storage, not as an energy source, unless future technological developments (contrary to the conservation of the baryon number, such as a CP violation in favour of antimatter) allow the conversion of ordinary matter into anti-matter. Theoretically, humans may in the future have the capability to cultivate and harvest a number of naturally occurring sources of antimatter.[21][22][23]

Perhaps a more exotic means to generate usable energy would be to feed a stellar mass into a black hole, and collect photons emitted by the accretion disc.[25][26] Less exotic would be simply to capture photons already escaping from the accretion disc, reducing a black hole's angular momentum; this is known as the Penrose process.

Star lifting is a process where an advanced civilization could remove a substantial portion of a star's matter in a controlled manner for other uses.

There are many historical examples of human civilization undergoing large-scale transitions, such as the Industrial Revolution. The transition between Kardashev scale levels could potentially represent similarly dramatic periods of social upheaval, since they entail surpassing the hard limits of the resources available in a civilization's existing territory. A common speculation[27] suggests that the transition from Type 0 to Type I might carry a strong risk of self-destruction since, in some scenarios, there would no longer be room for further expansion on the civilization's home planet, as in a Malthusian catastrophe. Excessive use of energy without adequate disposal of heat, for example, could plausibly make the planet of a civilization approaching Type I unsuitable to the biology of the dominant life-forms and their food sources. If Earth is an example, then sea temperatures in excess of 35 °C (95 °F) would jeopardize marine life and make the cooling of mammals to temperatures suitable for their metabolism difficult if not impossible. Of course, these theoretical speculations may not become problems possibly through the applications of future engineering and technology. Also, by the time a civilization reaches Type I it may have colonized other planets or created O'Neill-type colonies, so that waste heat could be distributed throughout the planetary system.

Many extensions and modifications to the Kardashev scale have been proposed.

Types 0, IV, and V Kardashev rating: The most straightforward extension of the scale to even more hypothetical Type IV beings who can control or use the entire universe or Type V who control collections of universes. This would also include Type 0 civilizations, who do not rank on the Kardashev scale. The power output of the visible universe is within a few orders of magnitude of 1045 W. Such a civilization approaches or surpasses the limits of speculation based on current scientific understanding, and may not be possible.

Zoltán Galántai has argued that such a civilization could not be detected, as its activities would be indistinguishable from the workings of nature (there being nothing to compare them to).[28]

In his book Parallel Worlds, Michio Kaku has discussed a Type IV civilization that could harness "extragalactic" energy sources such as dark energy.[29]

Kardashev alternative rating characteristics: Other proposed changes to the scale use different metrics such as 'mastery' of systems, amount of information used, or progress in control of the very small as opposed to the very large.

Planet mastery (Robert Zubrin): Metrics other than pure power usage have also been proposed. One is 'mastery' of a planet, system or galaxy rather than considering energy alone.[30]

Information mastery (Carl Sagan): Alternatively, Carl Sagan suggested adding another dimension in addition to pure energy usage: the information available to the civilization.

He assigned the letter A to represent 106 unique bits of information (less than any recorded human culture) and each successive letter to represent an order of magnitude increase, so that a level Z civilization would have 1031 bits.

In this classification, 1973 Earth is a 0.7 H civilization, with access to 1013 bits of information.

Sagan believed that no civilization has yet reached level Z, conjecturing that so much unique information would exceed that of all the intelligent species in a galactic supercluster and observing that the universe is not old enough to exchange information effectively over larger distances.

The information and energy axes are not strictly interdependent, so that even a level Z civilization would not need to be Kardashev Type III.[5]

Microdimensional mastery (John Barrow):John D. Barrow, going by the fact that humans have found it more cost-effective to extend any abilities to manipulate their environment over increasingly smaller dimensions rather than increasingly larger ones, reverses the classification downward from Type I-minus to Type Omega-minus:

Type I-minus is capable of manipulating objects over the scale of themselves: building structures, mining, joining and breaking solids;

Type II-minus is capable of manipulating genes and altering the development of living things, transplanting or replacing parts of themselves, reading and engineering their genetic code;

Type III-minus is capable of manipulating molecules and molecular bonds, creating new materials;

It has been argued that, because we cannot understand advanced civilizations, we cannot predict their behavior. Thus, the Kardashev scale may not be relevant or useful for classifying extraterrestrial civilizations. This central argument is found in the book Evolving the Alien: The Science of Extraterrestrial Life.[32]